1. Field of the Invention
The present invention relates generally to audio amplifiers, and relates more particularly to powered or active amplifier speaker combinations with electrical isolation features.
2. Description of Related Art
Powered speakers essentially have a power amplifier suitably mounted to the speaker cabinet for driving the speakers. The power amplifier is matched to the capability of the speakers, or transducers, as a group, and a conventional passive crossover system is used between the various transducers. That is, the transducers are coupled to the amplifier output with individualized compensation components that are passive. Typically, the input is an earth ground-referenced, low signal-level input. The input line power includes an earth ground reference and the amplifier output connections are not user-accessible. The input line power is typically a utility voltage input for powering the amplifier.
Active speakers are different from powered speakers in that each transducer, for example, low, mid, and high frequency speakers, has its own amplifier specifically matched to it to optimize individual transducer performance. In addition, an optimized electronic crossover splits the audio signal more accurately, e.g., through active filters, and with less distortion than conventional passive crossovers. Moreover, the electronic crossover can include phase-delay correction to properly time-align the transducers, thus eliminating the need to have all the transducers physically aligned in the speaker cabinet. The phase delay correction permits construction of more compact cabinets without compromising performance. Like powered speakers, active speakers have a utility power input, isolated audio inputs, and the amplifier outputs are not user accessible.
In a conventional audio system with separate amplifiers and passive speaker components, the inputs and outputs of the amplifier are isolated from the line input connection to comply with electrical safety regulations. This means that the “common” or ground terminals are at earth ground, and there is no galvanic connection between any user-accessible connection and the line input. These conventions are maintained for both signal input and speaker output connections, when both are user accessible.
Since power amplifiers typically have one or more DC busses internally, galvanic isolation is conventionally provided in the power supply section for ease of design. Designs using conventional linear power supplies typically have line-frequency transformers that provide isolation. Higher-powered amplifiers use switching power supplies with isolation provided in the high-frequency transformer. Switching power supplies, however, can generate significant electrical noise (EMI), and are therefore not commonly used, except in the highest power professional audio amps.
Many powered and active speakers today use relatively conventional power supply topologies, so that the input and output of the amplifier are isolated. The use of conventional power supplies adds extra expense and weight, especially in the case of linear transformers. The linear transformers typically have a greater VA rating than the total output power of the amplifier(s). Moreover, most amplifiers use tape-wound toroidal power transformers because they emit far less leakage magnetic field than standard transformers, and therefore induce less “hum” into the system. However, the toroidal transformer construction is more expensive than conventional bobbin-wound transformers, and therefore adds cost to the system.